////   This file contains the C# code from Program 9.26 of//   "Data Structures and Algorithms//    with Object-Oriented Design Patterns in C#"//   by Bruno R. Preiss.////   Copyright (c) 2001--2002 by Bruno R. Preiss, P.Eng.  All rights reserved.////   http://www.brpreiss.com/books/opus6/programs/pgm09_26.txt//public class ExpressionTree : BinaryTree{    private class ExpressionVisitor : AbstractPrePostVisitor    {	StringBuilder builder = new StringBuilder();	public override void PreVisit(object obj)	    { builder.Append("("); }	public override void InVisit(object obj)	    { builder.Append(obj); }	public override void PostVisit(object obj)	    { builder.Append(")"); }	public override string ToString()	    { return builder.ToString(); }    }    public override string ToString()    {	ExpressionVisitor visitor = new ExpressionVisitor();	DepthFirstTraversal(visitor);	return visitor.ToString();    }    // ...}